drm/drm_stub: Remove unnecessary null check before kfree.

kfree on a null argument is a no-op.
Silences the following smatch warning:
drivers/gpu/drm/drm_stub.c:496 drm_put_dev() info:
redundant null check on dev->devname calling kfree()

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>